Yep, all you have to do is copy and paste the stage that you want, use your .rel over Castle Seige and rename all the copies to STGEMBLEM_00, STGEMBLEM_01 and STGEMBLEM_02
For example, if you had an FD stage you wanted over Castle Seige, copy and paste the STGFINAL.pac and rename the copies to the names above.
I was talking more along the lines of a bunch of different (FD for example) stages over each file.
So for example, over Spear Pillar, I have Lensho's Mt. Pyre for one, Tin Roof for another, and this other guy's Legendary Valley over the third. Since all of these stages were built over Battlefield. I did the same thing with like Shadow Moses and Mushroom Kingdom.
Is there a way to have different (though based off of the same stage) stages over Castle Siege?